Pigment compositions use pigments having excellent light fastness as coloring materials, and utilization of the excellent light fastness of the pigments allows the pigment compositions to be used in various indoor and outdoor applications such as the field of coatings for automobiles and buildings, the field of printing inks such as offset inks, gravure inks, flexographic inks, and silk screen inks, and the field of aqueous inks for inkjet recording.
Among the pigment compositions, aqueous inks that use water as their main solvent have an advantageous feature in that the risk of fire, which is high for solvent inks, can be reduced. Such aqueous inks are the mainstream of inks particularly for inkjet recording.
A pigment is insoluble in water and is used as an aqueous pigment dispersion prepared by dispersing the pigment in an aqueous medium. Accordingly, methods for stably dispersing a pigment in an aqueous medium have been studied.
Known examples of such methods include: a method including well-mixing a synthetic resin having a specific acid value, a base, and a coloring agent using an agitator or a dispersing device in the presence of an organic solvent such as a ketone-based solvent or an alcohol-based solvent to dissolve or disperse the coloring agent and render the synthetic resin self-water dispersible and then mixing the colored resin solution with an aqueous medium to disperse small droplets of the self-water dispersible resin solution containing the coloring agent in the aqueous medium (see, for example, paragraph 0024 in PTL 1 and paragraph 0025 in PTL 2); a method in which a block polymer compound having a hydrophobic segment and a hydrophilic segment including hydrophobic units and hydrophilic units is used as a pigment dispersant (see, for example, PTL 3); and a method in which an (An-Bm) block polymer is used as a pigment dispersant, the (An-Bm) block polymer being an A-B block polymer wherein A is styrene, B is acrylic acid, the degree of polymerization of A is about 5 to about 50, and the degree of polymerization of B is about 70 to about 800 (see, for example, PTL 4).
One challenge of an aqueous pigment dispersion is to reduce the amount of coarse particles that may be formed in the dispersion. The coarse particles are particles having diameters much larger than the average particle diameter of the aqueous pigment dispersion obtained and may be aggregates of undispersed particles of the pigment and debris of the pigment formed during dispersion or aggregates of the polymer used as a pigment dispersant.
The coarse particles inhibit the formation of a uniform coating surface during coating or printing. In a printing method in which an ink such as an aqueous ink for inkjet recording is ejected from nozzles of an inkjet head, the coarse particles cause clogging of the nozzles. Particularly, since the resolution of inkjet printers is increasing in recent years, the density of nozzles of inkjet heads is increasing, and the size of liquid droplets is decreasing. In other words, the diameter of the nozzles for ejecting the ink is decreasing, and their density is increasing (see, for example, PTL 5). As the diameter of the nozzles decreases, the allowable size of foreign matter decreases, and this causes an increase in the frequency of the occurrence of clogging of the nozzles. Specifically, a problem occurs in that inks applicable to conventional inkjet printers cannot be used for a printer equipped with a newly developed high-resolution inkjet head.
The method described in PTL 1 is a good method for producing an aqueous ink for inkjet recording. However, in order to provide an ink that can cope with a reduction in the diameter of nozzles and an increase in their degree of integration in recent years, there is still room for consideration.